The answer would be letter D. <em>The hideous and revolting conduct of the past and present</em>
Letter A makes no sense with the text.
C, although a somewhat possible conclusion, is not the message the author attempts to transmit.
B, although maybe the main argument to sustain the author's point, it's still not the main idea he aims to pass.
The author intent is to show what the Fourth of July looks like from the slaves' point-of-view. For that, he reminds people of the disparity between them (free white people) and slaves, of how the feelings of liberty and justice do not encompass all. And because of it, this day may be a celebration occasion to them, but to slaves, it's a 'mourning' occasion, with nothing to rejoice or be proud of. Finally, he concludes by stating "<em>Whether we turn to the declarations of the past, or to the professions of the present, the conduct of the nation seems equally hideous and revolting.</em>" - which is the answer to the question.
